collections: Add dummy MutableMapping ABC.

Actually should be in collections.abc since Python 3.3, but for compatibility
still present directly in collections, where cross-version software digs for
it.

collections/collections/__init__.py

@@ -11,3 +11,6 @@ try:
     from .deque import deque
 except ImportError:
     pass
+
+class MutableMapping:
+    pass